1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an in plane switching mode LCD device, in which a circuit for a common voltage and additional elements thereof are omitted to reduce a flicker.
2. Discussion of the Related Art
Recently, various display devices serving as interface between a human being and information through various kinds of visual information are used. In particular, LCD devices are widely used as the next generation display devices, replacing conventional cathode ray tubes (CRT) because of the advantages of LCD devices, such as a high picture quality, a low power consumption, a light weight, and the like.
LCD devices use the optical anisotropy of liquid crystals to display an image by controlling the transmittance of light supplied from a light source. The transmittance of the light is controlled by applying an electric field to liquid crystals contained between a thin film transistor array substrate and a color filter substrate, thereby rearranging the liquid crystals.
The LCD device is divided into a twisted nematic (TN) mode LCD device and an in plane switching (IPS) mode LCD device depending on a driving mode of liquid crystal molecules.
The TN mode LCD device includes a thin film transistor array substrate including a pixel electrode, a color filter array substrate including a common electrode, and a liquid crystal layer disposed between the two substrates. The liquid crystal layer is arranged depending on a vertical electric field generated between the common electrode and the pixel electrode. The pixel electrode is formed per unit pixel, and the common electrode is formed on an entire surface of the color filter substrate.
The TN liquid crystal is rearranged by a vertical electric field of the pixel electrode on the thin film transistor array substrate and the common electrode formed on the color filter substrate. Accordingly, the light transmittance of the TN liquid crystal changes in accordance with the viewing angle in all directions, which limits the fabrication of large area LCD devices. That is, in the TN mode LCD device of which liquid crystal is rearranged by the vertical electric field, the light transmittance is symmetrically distributed according to a viewing angle in right and left directions but is asymmetrically distributed according to a viewing angle in up and down directions. For this reason, image inversion is generated in up and down directions, thereby narrowing the viewing angle.
In order to solve the above problem, an in-plane switching (IPS) mode LCD device of which liquid crystal is driven by a horizontal electric field has been proposed.
The IPS mode LCD device includes a thin film transistor array substrate including common and pixel electrodes, a color filter substrate including common electrodes, and a liquid crystal layer formed between the two substrates, wherein the liquid crystal layer is rearranged by the horizontal electric field between the common electrode and the pixel electrode. The common and pixel electrodes are alternately formed at constant intervals for unit pixel.
The IPS mode LCD device enhances viewing angle characteristics such as contrast ratio, gray inversion, and color shift, as compared to an LCD device where the liquid crystal is driven using a vertical electric field. Therefore, since the IPS mode LCD device obtains a wider viewing angle, it is widely used for the fabrication of LCD devices with a large display area.
Hereinafter, the IPS mode LCD device constructed as above will be described with reference to FIG. 1 and FIG. 2.
FIG. 1 is a plan view illustrating a thin film transistor array substrate in a general IPS mode LCD device, and FIG. 2 is a circuit diagram illustrating an equivalent circuit of FIG. 1.
As shown in FIG. 1 and FIG. 2, the IPS mode LCD device includes a plurality of gate lines GL1 to GLn arranged on a thin film transistor array substrate in a horizontal direction, and a plurality of data lines DL1 to DLm arranged on the substrate in a vertical direction to cross the gate lines GL1 to GLn. A plurality of pixels P1 (n×m) are defined at each crossing between the gate lines GL1 to GLn and the data lines DL1 to DLm. Each pixel P1 is provided with a pixel electrode 11 and a switching device T1.
The thin film transistor array substrate is provided with a plurality of common voltage lines CL1 to CLn for supplying a common voltage Vcom to the pixels P1. The respective common voltage lines CL1 to CLn are adjacent to the respective gate lines GL1 to GLn and are extended along the gate lines GL1 to GLn.
Generally, the switching device T1 includes a thin film transistor. A source electrode of the switching device T1 is connected with the data lines DL1 to DLm, its gate electrode is connected to the gate lines GL1 to GLn, and its drain electrode is connected to the pixel electrode 11.
The pixel P1 is provided with not only the pixel electrode 11 but also a common electrode 13. The common electrode 13 is electrically connected to the common voltage lines CL1 to CLn, and is applied with the common voltage Vcom. The common electrode 13 is arranged in the pixel P1 in an alternating pattern and parallel with the pixel electrode 11.
In the aforementioned IPS mode LCD device, when a scan signal is sequentially applied to the gate lines GL1 to GLn from a gate driver, the switching devices T1 connected to gate electrodes of corresponding gate lines GL1 to GLn are turned on by the voltage of the scan signal. At this time, an image signal output from a data driver is applied to the pixel electrode 11 through the source electrode of the switching device T1. A data voltage according to the image signal is applied to the pixel electrode 11.
The common electrode 13 is applied with the common voltage Vcom through the common voltage lines CL1 to CLn, and generates a horizontal electric field in the pixel P1 area along with the pixel electrode 11 arranged in parallel therewith. The liquid crystal inside the pixel P1 is rearranged by the horizontal electric field. Arrangement of the liquid crystal changes based upon the intensity of the electric field, thereby varying the transmittance of the light supplied from a lamp. Since the common voltage lines CL1 to CLn are electrically connected to one another, the same common voltage is applied to each common electrode 13 through the common voltage lines CL1 to CLn.
However, the aforementioned IPS mode LCD device has several problems. That is, because the common and pixel electrodes are both arranged on the same thin film transistor array substrate for plane driving of the liquid crystal, an aperture ratio is low and luminance is reduced.
Also, when the scan signal is changed to a low potential, the data voltage is dropped at a certain size due to coupling between the gate electrode and the drain electrode of the switching device, whereby a flicker occurs, which causes images of irregular gray level. If such a flicker occurs in every pixel of the LCD panel, picture quality may be seriously deteriorated.